In the oxidation of chlorinated hydrocarbons, aluminum oxide in the form of particulate alumina is one of the materials of choice. There is an upper use temperature limit for the gamma form of alumina. Alumina is suitable by itself for the oxidation of hydrocarbons, however activated forms are available which contain one or more metal oxides, platinum, platinum/metal oxide, and others. Specific examples include Cr.sub.2 O.sub.3 CuO/Al.sub.2 O.sub.3, Pt/Al.sub.2 O.sub.3, MnO.sub.2 /Cr.sub.2 O.sub.3 /Al.sub.2 O.sub.3, and those containing oxides of cobalt, nickel, vanadium, and molybdenum.
Commercial catalytic oxidation processes have been adapted for disposal of waste organic solvents, ground water pollutants, synthetic by-products, incinerator flue exhaust and automotive exhausts. In the large scale catalytic oxidation of chlorinated hydrocarbons, fuel value is typically recovered as steam and chlorine is recovered as HCl and/or Cl.sub.2. Various approaches are taught in the patent and scientific literature.
Jamal Eden disclosed in British Patent No. 1,506,238 a Cr.sub.2 O.sub.3 impregnated alumina, silica or mixture as a fixed or fluidized catalyst for treating the by-product waste stream from the oxychlorination of ethylene. The oxidized exit stream, free of chlorohydrocarbons, and containing HCl, can be re-used in the oxychlorination process. The catalyst comprised 10-50% Cr.sub.2 O.sub.3 on 90-50% alumina. The temperature of the catalyst bed was maintained at from 300.degree. C. to 450.degree. C. Eden prepared the catalyst by impregnating alumina with aqueous Cr(NO.sub.3).sub.3.9H.sub.2 O or CrCl.sub.3.6H.sub.2 O, drying over hot air, and calcining for approximately 16 hours at 540.degree. C. The catalyst preparation of Eden is consistent with the conventional teaching that catalyst calcination temperature should be as high as the intended use temperature, and typically about 50.degree. to 100.degree. C. higher than the intended use temperature.
Ernest Johnston has disclosed in U.S. Pat. No. 3,989,807 the use of fluidizable Cr.sub.2 O.sub.3 /alumina catalyst for recovering chlorine values from direct liquid injection of chlorinated organic compounds mixed with air. In Johnston's process elemental chlorine (Cl.sub.2) was recovered rather than the HCl intermediate. Johnston's catalyst preparation consisted of impregnation of chromium salt or oxide on the support, followed by drying and optionally conditioning at 350.degree. C.-500.degree. C. Johnston suggested generally a chromium metal content of from 0.1 to 20 weight percent, preferably 0.5 to about 10 percent, on a support having a surface area at least 50 m.sup.2 /g, preferably at least about 200 m.sup.2 /g.
William Hunter, et al. disclosed in U.S. Pat. No. 4,330,513 a catalytic oxidation process using conventional chromium on alumina. The catalyst comprised 15% to 25% by weight chromic oxide on a suitable attrition resistant carrier having a particle size ranging from 500 microns up to 0.25 inches. Hunter et al. suggested a catalyst preparation by conventional techniques with a preferred embodiment prepared by forming large preformed shapes such as pellets from an extruded support followed by calcining the bare support. Aqueous chromic acid was then applied at 120.degree. F. followed by drying at 250.degree. F. and calcining at 1300.degree. F. (704.degree. C.).
The deactivation of commercial chromia-alumina catalysts over long term exposure of streams containing 500 ppm of C.sub.1 -C.sub.2 chlorohydrocarbons and mixtures has been studied. See S. K. Agarwal, J. J. Spivey and J. B. Butt Catalyst Deactivation During Deep Oxidation of Chlorohydrocarbons, Applied Catalysis A: General, 82, (1992), pp. 259-275. The authors found that during long term use-on-stream the reaction zone temperature needed to be increased in order to maintain high conversion rates. The authors observed steady deactivation for a mixed stream of chlorinated and non-chlorinated hydrocarbons. The temperature needed to be raised from 305.degree. C. to 418.degree. C. over 210 days to maintain conversion at or above 99%. The volatilization of chromium appears worse for chlorinated hydrocarbon feedstreams. The authors speculated that the volatilizing of chromium oxychloride may be beneficial in continuously exposing fresh catalyst surfaces.
In commercial scale catalytic oxidation of chlorinated materials, the maximum operating temperatures are limited by the optimal temperature range for the chosen catalyst as well as the corrosion resistance inherent in the metals used for the equipment. For example, certain economical nickel alloy steels undergo catastrophic corrosion in the presence of HCl at or above 530.degree. C. Increasing the operating temperature of the reaction zone approaching this temperature will lead to higher corrosion rates. It would therefore be desirable from an economic standpoint to maintain very high conversion of 99% or higher of feedstocks over long periods of time without risking increased rates of corrosion. In view of the recognition that conventional chromium catalysts deactivate during long term exposure to chlorohydrocarbons, there is a need for eliminating deactivation or substantially reducing the rate at which a chromium catalyst loses its activity level.
The art related to supported oxidation catalysts recognizes loss in catalyst activity is caused by physical attrition, and transformations in the support material leading to a loss in surface area. Loss of active metal oxide follows with time-on-stream from physical attrition and volatilization in the form of chromium oxychloride which is increased in the presence of chlorinated feedstocks. The art suggests that in some processes, sloughing off of catalyst surface is beneficial for maintaining activity. The loss of the active metal from catalyst supports may be beneficial from a technical standpoint, however from an environmental view, metal loss is problematic because the effluent streams from catalytic oxidation processes must conform to local and state emissions regulations. The hexavalent chromium in chromium oxychloride (CrO.sub.2 Cl.sub.2), for example, is listed as a carcinogen; and trivalent chromium is considered a toxic substance and there are stringent upper limits on their concentration in discharged waste. Therefore, it would be desirable to provide reduced chromium emissions in the waste effluent of catalytic oxidation processes.
The inventors have investigated the changes in metal oxide catalysts over time-on-stream in the catalytic oxidation of chlorinated hydrocarbons and have discovered a method of making a chromium impregnated catalyst which unexpectedly shows high stability and reduced chromium volatilization during long term use. Therefore the catalyst of the present invention is not only more economical in use but is also more environmentally friendly.